A hot rolled steel sheet is usually used for a member of an automobile or a truck, such as a chassis or a wheel, and is required to have bendability and high fatigue durability.
As a method for increasing the formability and fatigue characteristics of a hot rolled steel sheet, the following methods are disclosed. As shown in Patent Documents 1 to 3, there are methods for dispersing structures including hard martensite in a metal structure mainly including soft ferrite so as to make a so-called dual-phase steel. In these methods, an alloy element such as Si or Al, which has an effect of promoting the formation of ferrite during cooling after final rolling in a hot rolling process, is added.
However, in a case in which steels disclosed in Patent Documents 1 to 3 are manufactured in a real operation line, in some cases, favorable bending fatigue characteristics are not obtained stably. Further, as for a steel to which Si is added, unfortunately, there are cases in which coating corrosion resistance obtained after electrodeposition coating (hereinafter also simply referred to as “coating corrosion resistance” or “corrosion resistance after coating”) is not secured and cases in which bending fatigue characteristics are not obtained as expected because the surface roughness of a steel sheet becomes large.
Further, a steel sheet that is used for chassis or wheels of automobiles or trucks is also required to have fatigue characteristics of a punched part. This is because the roughness of an end surface formed by punching with shears or a punch usually has a larger roughness than the surface of a steel sheet, and the punched end surface becomes the position where a fatigue crack is generated preferentially.
As a method to solve the above problems, for example, Patent Documents 4 and 5 disclose a high-strength hot rolled steel sheet in which damage of the punched end surface is prevented.
Further, Patent Documents 6 and 7 disclose steel sheets having high notch fatigue strength. In the steel sheets, the main structure is ferrite and bainite.
Patent Document 8 discloses a method for largely decreasing the roughness of the punched fracture surface and notably preventing the generation of a fatigue crack from the punched part by making a structure in which appropriate amounts of martensite and retained austenite are dispersed while the strength is increased by using precipitation strengthening of an alloy carbide using, as a base, a metal structure including ferrite as a main phase.
Furthermore, Patent Document 9 discloses a steel sheet having high coating adhesion and punching fatigue characteristics while the strength is increased by using, as a metal structure including a ferrite phase as a main phase, a martensite and bainite structure.